Most cell surface and secreted proteins are modified by covalently-linked glycans which are essential mediators of biological processes such as protein folding, cell signaling, fertilization, embryogenesis, and cellular proliferation. Additionlly, abnormal glycosylation patterns of cell surface glycans have been shown to play a role in the development and progression of cancer. There is an expanding amount of literature that demonstrates that aberrant glycosylation of proteins offers the possibility to develop more reliable cancer biomarkers for early diagnosis. The ability of cells to generate information rich glycans has created a new field of research termed glycomics, which seeks to identify and understand the processes involved in the formation of cell type and developmental stage specific oligosaccharide patterns. Key glycomic technologies include mass spectrometric or HPLC profiling of glycan structures, glyco-gene microarrays for measuring expression levels of glycoenzymes and glycan binding proteins, and lectin arrays for screening glycan-protein interactions. Although the use of these technologies is beginning to show the functional significance of cell type specific glycosylation, progress is hampered by the lack of well-defined complex oligsaccharide standards. In many cases, well-defined oligosaccharides can only be obtained by chemical- or enzymatic approaches. Although tremendous progress has been made, complex oligosaccharide synthesis remains very time consuming and it is not uncommon that the preparation of a single well-defined derivative can take as much as a year. The need for more efficient approaches for oligosaccharide synthesis has stimulated the development of chemo-enzymatic methods in which a synthetic oligosaccharide precursor is modified by a range of glycosyl transferases to give a more complex compound. However, a serious limitation of this approach is that it provides only access to symmetrical branched oligosaccharides. The scope of chemoenzymatic synthesis is further limited by a lack of readily available glycosyltransferases. The aim of this project is to develop a novel chemoenzymatic methodology that will give easy access to libraries of well-defined symmetrical and asymmetrical branched oligosaccharides found on tumor cells. In collaboration with the alliance of Alliance of Glycobiologists for Detection of Cancer targets will be selected that will be prepared by this new methodology. The new complex oligosaccharides will be employed as standards for mass spectrometry to identify exact structures of tumor associated glycans. Furthermore, the glycans will be employed for glycan microarray development to probe carbohydrate protein interactions and profile serum samples of cancer patients for antibodies against tumor associated glycans that can have diagnostic value.